Abstract : Selenium Se is a chalcogen element with a narrow window between essentiality and toxicity. The toxicity is mainly related to the chemical speciation that Se undergoes under changing redox conditions. Se oxyanions, namely selenite SeIV, SeO32- and selenate SeVI, SeO42-, are water-soluble, bioavailable and toxic. In contrast, elemental selenium, Se0, is solid and less toxic. Nevertheless, Se0 nanoparticles are potentially harmful as particulate Se0 has been reported to be bioavailable to filter feeding mollusks e.g. bivalves and fish. Furthermore, Se0 is prone to re-oxidation to toxic SeO32- and SeO42- when discharged into aquatic ecosystems. Biogenic Se0 under investigation was produced by the reduction of Na2SeO4 under anaerobic conditions using a mixed bacterial inoculum anaerobic granular sludge and through the reduction of Na2SeO3 under aerobic conditions using a pure microbial culture Pseudomonas moraviensis stanleyae, a novel strain identified and characterized for the first time herein. Both types of Se0 showed strong colloidal stability within the 2-12 pH range. The colloidal stability is caused by the negatively charged -15 mV to -30 mV biopolymer layer covering biogenic Se0 particles and by their nanometer size. The particle size of Se0 produced by anaerobic granular sludge ranged between 50 and 300 nm, with an average size of 166 nm. Conversely, the Se0 particles produced by Pseudomonas moraviensis stanleyae are characterized by a lower diameter ~ 100 nm.The solid-liquid separation potential of Se0 was assessed by centrifugation, filtration, coagulation-flocculation and electrocoagulation. While all approaches can bring about Se0 removal from suspension with various degrees of success, electrocoagulation using iron sacrificial electrodes showed the highest removal efficiency 97%. Because biogenic Se0 is harmful to the environment, appropriate measures must be implemented for the solid-liquid separation using an efficient technology